All disk drives require some means for determining the radial and circumferential position of the read/write heads over the disks so that the heads can be accurately positioned over any desired track and sector. Typically, this is accomplished by placing servo information on one or more of the disk surfaces for use by magnetic or optical heads in determining their positional orientation over the disk. In sector-servo (also known as embedded servo) disk drives, the servo information is interspersed with data on each disk surface. This approach has the advantage of providing the positioning information close to the data sectors it identifies, thereby eliminating sources of track misregistration which otherwise tend to limit track density. However, a disadvantage of the sector servo approach is that it incurs additional overhead in order to permit transitions between data regions and servo regions and to distinguish data regions from servo regions.
Much attention has been focused in recent years on reducing the overhead associated with sector servo architectures. One approach, known as the no-ID format, is taught by Hetzler in copending U.S. patent application 07/727,680, filed Jul. 10, 1991. No-ID disk drives use servo sectors in combination with a defect map to identify the data sectors and completely eliminate the use of an ID region. Using the no-ID format, each sector on a track is composed of two regions: a servo region and a data region. The servo sectors are located using a servo ID mark or address mark. ECC may be added to track ID information to provide a more robust servo pattern. Each data sector is identified by its cylinder, head and servo sector number counted from an index location. This format is the same for substantially all sectors on all tracks of the disk.
A second strategy which has been used to improve recording density in sector servo disk drives in recent years is known as zone bit recording (ZBR), as taught by Hetzler in U.S. Pat. No. 5,210,660. In ZBR disk drives, the disk is divided into multiple zones oriented in the radial direction. Each zone is comprised of a set of tracks. Since tracks in the outer zones are longer than those in the inner zones, the tracks in the outer zones may store more data than the tracks in the inner zones. Typically, data is stored in sectors, each of which has the same number of data bytes. In this configuration, the additional capacity in the outer zones is utilized by having a larger number of data sectors on each track in the outer zones. This results in the number of data sectors per track varying from zone to zone. In order to provide a constant servo sampling rate for all zones, a single fixed number of servo sectors is used across the entire disk. The combination of a varying number of data sectors per track and a fixed number of servo sectors per track can result in some of the data sectors being split by servo sectors. An example of a disk formatted according to the Hetzler teaching is shown in FIG. 1, where data recording disk 101 is split into three zones--102, 104, and 106. Each zone is comprised of a plurality of tracks 103. Each track has a number of data sectors 105 with associated ID fields 107. Various servo sectors, designated as 108, are shown interspersed with data sectors 105 around the disk. An index location 109 is shown, where the data sectors in each zone align with a servo sector 108. A portion of a track on the disk is shown expanded at 110. Four complete data sectors are shown (130, 122, 132 and 124), each with their associated ID field (140, 141, 142 and 143). Three representative servo sectors 125, 126 and 128 are also shown. As can be seen from this example, some data sectors will be split by servo sectors, and some data sectors will not start immediately following a servo sector. For example, data sectors 122 and 124 are split by servo sectors 126 and 128, respectively, while data sectors 130 and 132 are not split by servo sectors. Data sectors 122, 132 and 124 and associated ID fields start immediately after another data sector, rather than immediately following a servo sector.
The necessity of splitting data sectors and of having some data sectors that do not start immediately following a servo sector presents complications which heretofore have prevented the use of ZBR and no-ID together in the same disk drive. For instance, in No-ID disk drives, the physical location of a data sector is derived from the address mark field, which is also used to locate the servo sector. However, this technique is dependant on a fixed, constant one-on-one relationship between the locations of the servo sectors and the data sectors, a relationship which does not exist in a ZBR-formatted disk drive.
A technique has recently been introduced which addresses part of the problem by providing electronics to generate timing pulses to mark the locations of data sectors which are not necessarily adjacent to servo sectors and which may be split by servo sectors. The technique was introduced by AT&T in the ATT93C010 servo channel/multiprocessor chip which generates a start of data sector pulse for each data sector starting between two servo sectors. This is achieved through the use of programmable registers whose values are updated at every servo sector. Two values are required at each servo sector: the number of clocks (the length) from the prior servo sector to the start of the first complete data sector; and the number of data sectors which start before the next servo sector. Also, the system must know the number of clocks required for a full data sector, a value which is typically constant for each zone.
However, while the ATT93C010 is able to locate the start of a data sector, it cannot identify a data sector--that is, distinguish it from other data sectors, such as by computing its data sector number. In fact, it cannot even compute a partial data sector number for use in distinguishing a sector from others on the same part of a track. As such, it is insufficient for use in a disk architecture which must both locate and identify data sectors without using an ID field.
Another possibility for locating data sectors without using ID information is to add a servo-style address mark prior to each data sector. This approach ensures that each data sector can be located independently of a servo sector. However, it suffers several drawbacks. First, it does not allow the disk drive to be reformatted with a sector size different from the original sector size since the address marks must be written by a servo writer which permanently fixes the disk format. Second, this approach can increase the complexity of the servo write process, because additional steps may be required to create the additional address marks. Third, the address marks occupy space on the disk, increasing the overhead. Fourth, the address marks require a write-to-read recovery region between data sectors, further increasing the overhead. Finally, when used with a magneto-resistive read/write head and micro jog technology, each address mark must be reliably read in a partially off-track position during write operations. This requires guard bands at the zone boundaries, since the data address marks do not line up with one another from one zone to the next. The guard bands, of course, further increase the overhead penalty associated with this method.
Accordingly, there has existed a heretofore unmet need in the art for a sector architecture which effectively combines the ZBR and no-ID formats, which sector architecture enables the data recording head to locate and identify data sectors for read and write operations without resorting to an address mark and without requiring write-to-read recovery between adjacent data sectors.